Before we dive into the fundamental SI units, let's review measurement as a concept. When you measure something, you use an instrument or device to determine some physical quantity of an object. For example, you use a ruler to measure length, a scale to measure mass and a thermometer to measure temperature. Each of those instruments comes marked in standard units to make sure the measurement of one observer matches that of another observer. In theory, each standard unit would trace its lineage back to a single prototype -- the archetypal example of that particular unit.
In earlier versions of the metric system, the prototypes were physical objects, such as a standard meter stick or a standard kilogram bar. When the General Conference on Weights and Measures revamped the metric system in 1960, it replaced units based on physical objects with physical descriptions of the units based on stable properties of the universe. In fact, the only unit still defined by an object is the kilogram. (The International Prototype Kilogram is a shiny cylinder made of platinum and iridium, stored in an airtight jar in Sèvres.)
With that in mind, let's introduce the seven SI base units. The table lists each unit, the physical quantity that unit measures and the standard upon which the unit is based, as defined by the International Bureau of Weights and Measures.
If you don't fully understand the definition for each standard, don't worry. Instead of trying to picture two straight parallel conductors of infinite length or a cesium-133 atom vacillating between two hyperfine levels of its ground state, just remember this: The fundamental SI units (except for the kilogram) are based on immutable properties of the universe, and they are mutually independent. All other units in the modern metric system come by multiplying or dividing these base units. We'll get into that more in the next section.